Damaged Devices in a ZFS Storage Pool

The term “damaged” covers a wide variety of possible errors.
Examples include the following:

Transient I/O errors due to a bad disk or controller

On-disk data corruption due to cosmic rays

Driver bugs resulting in data being transferred to or from
the wrong location

A user overwriting portions of the physical device by accident

In some cases, these errors are transient, such as a random I/O error
while the controller is having problems. In other cases, the damage is permanent,
such as on-disk corruption. Even still, whether the damage is permanent does
not necessarily indicate that the error is likely to occur again. For example,
if an administrator accidentally overwrites part of a disk, no type of hardware
failure has occurred, and the device does not need to be replaced. Identifying
the exact problem with a device is not an easy task and is covered in more
detail in a later section.